It is advantageous to generate and detect microwave signals with frequencies as large as 110 GHz. This need is perpetuated by increasing traffic being carried at lower frequencies (i.e., <40 GHz), and advances in photonic and electronic technologies that provide for the generation of stable millimeter waves. A widely used component in radio frequency (RF) and microwave systems is the voltage-controlled oscillator (VCO). The ability to tune the frequency via a DC voltage is a key feature of VCOs. While VCOs may be deployed in the microwave and millimeter regime, there is a trade-off between wideband tunability and low phase noise of generated microwave signals. For example, a typical VCO having a 1.2 GHz bandwidth exhibits a phase noise of −108 dBc/Hz at a 10 kHz offset for a carrier frequency of 2.5 GHz. As a comparison, a wideband (20 GHz) VCO exhibits a phase noise of −82.76 dBc/Hz at a 50 kHz offset for a carrier frequency of 40 GHz. These characteristics highlight this fundamental trade-off between tunability and phase noise.
The ability to optically synthesize microwave signals (OSMS) is an attractive method because of the very large carrier frequencies involved, e.g. 192 THz for Datacom wavelength (1550 nm) lasers. One known expedient to OSMS is to heterodyne a pair of tunable lasers. However, because of the convolution of the corresponding optical fields at a photodetector, the greater deleterious phase noise of the two lasers will be transferred to the microwave signal. In order to reduce the undesirable effects of such phase noise, a known method is to phase lock the lasers, for example, by using an optical phase-locked loop (OPPL). Although this approach is successful in reducing the phase noise, it restricts the tuning bandwidth and tuning speed, both of which depend on the OPPL bandwidth
Other known systems for generating stable, low phase-noise microwave signals, use an optoelectronic oscillator (OEO). The topology of the OEO includes a continuous-wave (CW) laser, which is injected into an optoelectronic loop comprising an electro-optic (EO) modulator, and a long fiber delay, a photo-diode, an electronic amplifier and a filter. Due to the gain in the OEO loop, any inherent noise undergoes an amplification and, given the appropriate operating parameters, sustains stable oscillations. In the absence of an electronic filter, this system is typically “multimode,” with a frequency spacing between the modes determined by the length of the OEO loop. In addition, the long fiber delay provides a Q of sufficient magnitude necessary to reduce the linewidth of the microwave signal. OEO topologies have undergone significant improvements, such as the use of dual cavity configurations and accompanying dramatic reductions in size. The size reduction is achieved by replacing the long fiber delays with micro-cavities having extremely large Q factors, e.g. a whispering-gallery-mode (WGM) oscillator, micro-ring resonator, etc. Another modification of this system reduces the number of components by replacing the EO modulator and CW laser source with a single semiconductor laser (SCL). Recently, an optical injection OEO scheme has been implemented having a slave laser that functions as an electronic filter. However, such a design necessitates the use of an electronic amplifier, and long-term stability has not been addressed.
In view of the above, it would be advantageous to provide a system and methodology for generating optically synthesized microwave signals with broadband tunability having superior performance and low phase noise.